The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard. UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. The radio base station (RBS) in E-UTRAN is called evolved NodeB (eNB).
In radio access networks such as E-UTRAN, multiple-input and multiple-output (MIMO) is the name applied for the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO can be sub-divided into three main categories: pre-coding, spatial multiplexing, and diversity coding. Pre-coding is multi-layer beam-forming (i.e. shaping of the overall antenna beam in the direction of a target receiver) in a narrow sense or all spatial processing at the transmitter in a wide-sense. In conventional (i.e. single-layer) beam-forming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver input. The benefits of beam-forming are to increase the signal gain from constructive combining and to reduce the fading effect. In the absence of scattering, beam-forming results in a well defined directional pattern. When the receiver has multiple antennas, the transmit beam-forming cannot simultaneously maximize the signal level at all of the receive antennas and pre-coding is thus used. With pre-coding, the multiple streams of the signals are emitted from the transmit antennas with independent and appropriate weighting per each antenna such that the link throughput is maximized at the receiver output. Applying different weights to the transmitted signals of the different antennas can be expressed as applying a pre-coding vector to the transmitted signals. Pre-coder based spatial multiplexing can be seen as a generalization of pre-coder based beam forming with the pre-coding vector replaced by a pre-coding matrix. Hereinafter, the term pre-coding matrix will be used. The receiver estimates the channel and decides on a suitable pre-coding matrix from the code-book (containing the set of available pre-coding matrices), and feeds back the information about the selected pre-coding matrix to the transmitter.
FIG. 1 illustrates schematically a part of the radio access network in an LTE system (E-UTRAN). Based on measurements of the downlink reference signals of the different eNB antennas, the user equipment (UE) 150 decides on a suitable pre-coding matrix from the code-book corresponding to a certain beam 120. The index of the selected vector is reported to the eNB 110, and the eNB 110 can use this pre-coding matrix for the downlink transmission to the UE 150. In LTE the vector index is called Pre-coding Matrix Index (PMI).
Future radio access networks are expected, apart from many other aspects, to either require increased coverage, support of higher data rates or a combination of both. Increasing the network capacity may sometimes lead to a decreased coverage, but the usage of relays or repeaters is a good option for challenging the trade-off between the transmission range (coverage) and the end-to-end data range. The benefit of relays and repeaters stems from the fact that splitting a long distance into two roughly equidistant hops allows for an increased data rate.
Compared to relays, Radio Frequency (RF) repeaters are much simpler and thus cost less. An RF repeater (hereafter called just repeater) is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. They are suitable e.g. for extending coverage in a rural or suburban area. A feature that defines the repeater is that it receives, amplifies and retransmits the signal almost simultaneously.
FIG. 2 illustrates a cell 200 in an E-UTRAN with an eNB 210 and three repeaters 230a-c. The repeaters 230a-c amplify and retransmit the signals they receive from the eNB 210 and the UE 250. Remarkable coverage expansion can be achieved by employing many repeaters. However, a repeater amplifies both the received signal and the interference, which results in a higher inter-cell inference. With the use of on/off repeaters the inter-cell interference can be decreased. An on/off repeater is turned off (that is the retransmission of signals is turned off) when there are no UEs scheduled for transmission in its repeater area, and it does thereby not contribute to the interference. To control the on and off state of the repeaters, an RBS has to distinguish the repeater selection for each UE, i.e. which one of the repeaters that the UE is using for its communication. Based on the repeater selection of the different active UEs, the RBS can make decisions on what repeaters to turn off jointly with the scheduling decisions. Traditionally, the UE measures the reference signals from the repeaters, selects a preferred repeater based on these signals and feeds back the selected repeater index to the RBS via the repeater. The problem with this solution is that an extra uplink report is needed for this feedback, and that interaction signaling between repeaters and UEs is needed as well. This makes the integration of repeaters to existing networks complicated, and it also increases the signaling overhead.